Regarding a resin composition for encapsulation, which is used in encapsulation of optical semiconductor elements such as light emitting diode (LED) and the like, its cured product is required to have transparence. Therefore, in general, epoxy resin compositions obtained using a bisphenol A type epoxy resin, an alicyclic epoxy resin and the like epoxy resins and an acid anhydride as the curing agent are used widely for various purposes.
However, when the aforementioned epoxy resin composition is used as an encapsulating resin, internal stress is generated caused by the curing shrinkage at the time of the curing of the epoxy resin composition or by the distortion due to a difference in coefficient of linear expansion between the epoxy resin and the optical semiconductor element. As a result, the optical semiconductor element is deteriorated to cause a problem of, for example, when the optical semiconductor element is an emission element, reducing its brightness. Accordingly, as a method for reducing the aforementioned internal stress, a method has been proposed and partly realized, in which coefficient of linear expansion of an epoxy resin composition is approximated to that of an optical semiconductor element by adding inorganic powder having small coefficient of linear expansion such as silica powder or the like (e.g., see Reference 1).
However, the aforementioned method for approximating to that of an optical semiconductor element has a fatal disadvantage as an epoxy resin composition for encapsulating an optical semiconductor element, namely considerable reduction of light transmittance of the epoxy resin composition cured product. Thus, with the aim of solving the aforementioned disadvantage, for example, a method for reducing a difference in the index of refraction between a cured product comprising a resin component and glass powder as the inorganic powder has been proposed (cf. Reference 2).
However, even in a general case in which refraction index of a resin component is fitted to the average refraction index of transparent inorganic particles obtained by an immersion method or the like, the transparent inorganic particles have a distribution in the particle system by the aforementioned method, so that refraction index distribution among different particles cannot be disregarded and the light transmittance of the cured and formed product cannot be increased sufficiently because, when microscopically observed, a difference in the refraction index between the resin and transparent inorganic particles remains. In addition, even in case that the improvement of transmittance is attempted based on the average particle size, specific surface area is increased and reflection of light on the surface is increased when the average particle size is reduced, and when the average particle size is increased on the contrary, it is highly possible to cause a change in the internal refraction index distribution of each particle or to generate defects such as voids inside of the sealed and cured product, and these-cannot be disregarded and result in the reduction of light transmittance.
Reference 1: JP-A-11-74424
Reference 2: JP-A-2001-261933